In the control of fluid in industrial processes, such as oil and gas pipeline systems, chemical processes, etc., it is often necessary to reduce the pressure of a fluid. Adjustable flow restriction devices such as flow control valves and fluid regulators and other fixed fluid restriction devices such as diffusers, silencers, and other back pressure devices are utilized for this task. The purpose of the fluid control valve and/or other fluid restricting device in a given application may be to control flow rate or other process variables, but the restriction induces a pressure reduction inherently as a by-product of its flow control function.
Currently there are available fluid control valves containing a valve trim in the form of stacked disks forming a fluid pressure reduction device. With such a sliding stem fluid control valve it is desirable to have the flow capacity increase continually in a substantially linear manner as the valve is opened. In a proposed valve trim in the form of a cage with stacked disks, two different disks are used in an alternating configuration. In this configuration, a first flow disk has inlet and outlet slots cooperating with a plenum in a plenum disk, so that every other disk has no inlet openings to allow fluid flow to occur as the valve fluid control element travels through the stacked disks from a closed to an open position. Thus, a "stair step" in effect is created in the flow capacity characteristics for these prior valve trim stacked disk configurations, rather than the desired characteristic of having the flow capacity increase substantially linearly as the valve is opening.